Asian Clam - Annoyance or Nuisance?

September 2008 

Every parent knows their children have different cries. For example, there is the cry that says “I didn’t get my way”; the one that says “I stubbed my toe”; and the one that says “I just broke my arm”. Each cry elicits a different response ranging from a hug to a call to 9-1-1. When an ecosystem calls out, it is more difficult to determine what degree of response is appropriate.

For example, a new potentially invasive species was found recently in the Champlain Canal. While lake managers foresee the spread of many species, this species, Asian clam, had not been anticipated. Some invasives such as zebra mussels and alewives have caused tremendous changes in Lake Champlain’s ecosystem. Others, like the snail mud bythinia, appear relatively benign. Clearly, invasive species, like children’s cries, call for appropriately scaled responses, but the appropriate response in this situation was (and is) unclear.

Asian clams were first discovered in April in Fort Edward by employees of the New York State Museum out taking their dogs for a walk. The clams have been confirmed only on the Hudson side of the canal one lock below the Champlain side during subsequent surveys. In other words, the species is not yet in the Lake Champlain Basin, and is currently found in a relatively confined area where effective control measures could be attempted. Usually, by the time a new species is found there is nothing that can be done.

In appearance Asian clams and zebra mussels are quite different. Unlike zebra mussels, Asian clams have thick shells with prominent, raised ridges. They can grow to be up to an inch and a half in length, and have round, symmetrical shells. Zebra mussel shells are less round, and most native mussels are somewhat asymmetrical. Whereas zebra mussels attach to hard surfaces, Asian clams burrow into sediments, preferring sand and gravel. One similarity between zebra mussels and Asian clams is that they both have free-floating larval stages called veligers. The larvae of all native mussels attach to fish, using them to move from place to place.

The native range of Asian clams includes Southeast Asia, Africa, some Pacific Islands, and parts of Australia. They were first discovered in North America in 1924 in British Columbia, but it was not until they spread through the watershed of the Gulf of Mexico that they gained a reputation as an invasive species. 

Asian clams are well suited for spreading and becoming a nuisance. They are very hardy so they can easily survive transport in bait buckets or live wells of boats. They have been used as bait and for human food. Perhaps most importantly, they are hermaphroditic, so introduction of a single individual can result in a new population. They have been found in portions of the Erie and Barge Canal System in New York since at least 1998.

The principal impact of Asian clams in other watersheds has been economic. Their size and shell shape is almost perfect for clogging water intake pipes. Control of Asian clams at utilities and industrial plants to prevent clogging of pipes is estimated to cost up to $1-billion annually. Additionally, the clams can become so dense in irrigation canal systems that dredging is necessary to keep water flowing. They have also compromised concrete making. Since they are hardy, they can survive the initial mixing of the concrete and then make their way to the surface while it sets thus weakening the product. Asian clams may out-compete native species of clams, but definitive impacts have not been demonstrated. 

Control options for Asian clams involve use of chemicals – either chlorine or bromine or designated molluscicides. Each of these would have potential impacts on non-target species, though since the clam population appears to be somewhat restricted within the Champlain Canal the areal extent of treatment could be small. 

However, the ability of the species to impact Lake Champlain is open to question. The clams do not thrive at more northerly latitudes. Colder temperatures seem to slow their population growth. They are restricted to shallow water areas and do not appear to survive freezing temperatures. They only reproduce at water temperatures above 60 degrees Fahrenheit, a temperature the main lake does not generally reach until mid-June. In terms of impacts, the water intakes around the lake are already coping with zebra mussels; Asian clams may not be substantially different. Zebra mussels have also decimated the native mussel fauna of Lake Champlain, so it is unclear what further impacts Asian clams might have. Yet, the potential impacts amount to little more than speculation, so there is still substantial risk in allowing another species into the basin.

 Though parents can distinguish their children’s different cries, it sometimes is difficult to distinguish between the scream of a minor annoyance and that of a major emergency. Likewise, Asian clams send a message that is difficult to interpret. Nonetheless, it is incumbent upon lake managers to consider opportunities for eliminating Asian clam and the potential costs of inaction.